This invention relates generally to surface coatings for video display screens and is particularly directed to a multi-layer antistatic/antireflective coating which is applied to the video display screen by sputtering.
The outer surface of a display screen, or panel, of a video display device such as a cathode ray tube (CRT) is typically provided with a multi-layer coating which performs various functions. These functions include reducing light transmission through the glass display screen/outer coating combination for improved video image contrast. In addition, an inner layer of the surface coating is electrically conductive in order to shield viewers of the video display device from low frequency electromagnetic radiation and to dissipate electrostatic charge on the display panel to neutral ground. The coating also typically provides an antireflective capability to reduce light reflection from the display screen for ease in viewing a video image on the display screen.
Various approaches are employed in applying the multi-layer coating to the outer surface of a display screen. These techniques include spin and spray coating, sometimes referred to as the wet method, vacuum vapor deposition, and sputtering. Spin and spray coating methods have been widely used with materials containing Ag-Pd or Ag-Au colloid. While the coating thus formed possesses good electrical conductivity and relatively low light reflectance, it is of relatively low quality and involves high processing costs. These wet approaches also suffer from problems with reproducibility and control of the thickness of the coating and can be used with only a limited number of solvents. In addition, the spin and spray coating methods have problems when used with materials comprised of very fine (small) particles in providing uniform particle dispersion. These approaches also suffer from the possibility of environmental contamination.
The vacuum vapor deposition approach involves high temperature heat treatment and is thus energy intensive and more expensive than the wet coating approach. The sputtering approach has encountered difficulties in forming at high speed a stable SiO2 layer having a low refractive index for use in the antireflective layer. One approach involving sputtering for applying a light absorptive antireflective layer to a CRT display screen is disclosed in U.S. Pat. No. 5,691,044. This approach applies an inner layer of TiN to the surface of a glass substrate. The TiN layer suffers from instability at the high temperatures used for applying the multi-layer coating to the glass substrate. To improve the heat resistance of the TiN layer, an oxide barrier layer of metal nitride (TiN) is formed on the inner TiN layer. This approach requires various reacting gases such as N2 and O2 in the sputtering process which increases the cost and complexity of video display screen manufacture.
The present invention avoids the limitations of the prior art by providing a multi-layer antistatic/antireflective coating applied by sputtering to the outer surface of a video display screen which allows for precise control over the thickness of the multi-layer coating as well as its light transmission and electrical resistivity characteristics.
Accordingly, it is an object of the present invention to deposit a multi-layer coating on a video display screen in an environmentally clean manner while maintaining the desired optical and electrical characteristics of the coating.
It is another object of the present invention to form a two-layer antistatic and antireflective coating on the surface of a video display screen by sputtering.
Yet another object of the present invention is to provide a low cost, highly reproducible composition for, and a method for applying to the surface of a video display screen, a multi-layer antistatic antireflective coating having a wide range of components.
A still further object of the present invention is to provide a sputter coating technique for depositing a multi-layer coating on the surface of a video display screen which eliminates the need for a reactive gas and allows for close control of coating conductivity and reflectance by precise control of individual layer thickness.
The present invention contemplates a process for forming an antistatic/antireflective coating on an outer surface of a video display screen comprising the steps of: sputter-depositing on the outer surface of the video display screen an inner metallic antistatic layer having a precisely controlled thickness within a range of 18-35 nm, wherein a light refractive index of the inner antistatic layer is also precisely controlled within a range of 1.8-2.2; and sputter-depositing on the inner antistatic layer an outer antireflective layer having a precisely controlled thickness within a range of 110-140 nm, wherein a light refractive index of the outer antireflective layer is also precisely controlled within a range of 1.3-1.47. This invention also contemplates a multi-layer coating for a video display panel having the aforementioned composition as well as apparatus for sputter depositing a multi-layer antistatic/antireflective coating on the surface of a video display screen.